While much of the discussion herein relates to coating coins, this is merely one example of a substrate.
While much of the discussion herein relates to coating aluminum, this is meant to include coating aluminum or aluminum alloys.
There is an increasing demand for alternative low cost coinage materials which can maintain and/or increase security features and which are durable for use as circulation coins. Aluminum is one of the most promising candidate core materials due to its availability, low cost, lightweight, and excellent physical and chemical properties. Although aluminum is used as coin material in many countries, these coins have shown poor wear resistance and very low durability in the circulation environment. Some of the aluminum coins become dark and corroded shortly after being released for circulation. These corroded coins contaminate consumers' personal belongings. Henceforth, it is expected that plating metal(s) onto an aluminum substrate will provide enhanced wear resistance and more durability to the aluminum substrate.
It is well known that electro-plating on aluminum is much more difficult than plating on other metals such as steel or copper alloys. A major problem in plating aluminum is the difficulty in achieving good coating adhesion, particularly when using barrel plating. This is due to the fact that an aluminum oxide film tends to form on the aluminum surface immediately when the surface is exposed to air or water. This oxide film is detrimental to the plating process as it acts as a barrier to prevent direct metallic bonding between the plating and the aluminum core, thus resulting in poor adhesion between the plating and the substrate. Special pre-treatments have been developed to address the poor adhesion such as etching to remove the oxide, anodizing to create a rough surface, and pre-depositing to cover the oxidized surface, including electroless nickel plating and zincating in which an immersion deposit of zinc is produced.
In different simple zincating processes such as those described in ASTM B253-87 “Preparation of Aluminum Alloys for Electroplating”, different steps, including a zincating immersion solution containing elements such as zinc, copper, nickel, and complexing agents of cyanide and tartrate, are required for different aluminum alloys which results in an inconsistent adhesion between the plating and the substrate. For example, different zinc immersion solutions and procedures are recommended when sodium hydroxide and zinc oxide are mainly used with different additives under different conditions. Upon completion of the zincating step, either by the single or double zincating process, other pre-coating processes are applied. These processes include for example, cyanide copper striking, neutral nickel strike and electroless nickel, etc.
U.S. Pat. No. 6,692,630 to Morin et al. (“Morin”) discloses a two step zincating pretreatment for plating aluminum parts in a small barrel plating process. Morin describes a zincating process similar to the zincating process described in the ASTM B253-87 reference. According to Morin, the improvement in terms of better adhesion is due to the addition of potassium cyanide which acts as a complexing agent and a solution activator in the zincating process. Furthermore, no matter what metal plating on aluminum is undertaken, be it pure copper plating, copper alloy (brass or bronze) plating, or nickel plating, a copper layer strike is particularly emphasized, and is considered as a must for adhesion according to Morin. The copper layer strike in Morin, is plated using a cyanide copper striking bath.
In coinage materials, non-cyanide plating technology, as described in the U.S. Pat. No. 5,151,167 to Truong et al. (“Truong1”) and U.S. Pat. No. 5,139,886 to Truong et al. (“Truong2”), has been welcomed by many countries and is commercially available. Coins struck from the non-cyanide and multi-ply plating technology have been in circulation in many countries and have proved to be durable, secure, and cost competitive. In the multi-ply coin structure, nickel, copper and then another nickel layer are coated onto low carbon steel using an automatic loading and computer controlled process, which is cyanide free.